Hot Reloading in Ruby

Developer eXperience or DX is really important. Something that we use every day and almost forget is Hot Reload; in Ruby World, there is a cool gem that is appreciated: Spring for Hot Server Reload. So, in this article, we will see how to build a simple preloader in Ruby.

Polling

There are several techniques to achieve what we want. But the easiest one stays polling. Basically, what we will do here is create a watcher that polls over the files of our current directory. Then, our app will reload the files if necessary.

  class Watcher 

    def initialize(files)
      @relative_mtime = files.map do |file|
        File.mtime(file)
      end.max
    end

    def watch
      mtime_max = Dir.glob("**/*").map do |file|
        File.mtime(file)
      end.max

      if @relative_mtime < mtime_max
        @relative_mtime = mtime_max
        true
      else
        false
      end
    end
  end

App/Server Reloading

Now we can build our app, this app is simple but it could be a Rails app.

Here, you see that at the beginning, we require the Watcher, then a test1 file, and a test2 file. But with a little subtle thing. the first one is done with require and the other one with load.

We are going to see why after.

require 'watcher.rb'
require 'test1'
load 'test2.rb'

files = Dir.glob("**/*")

watcher = Watcher.new(files)

while 1
  Kernel.sleep 1
  begin
    Test1.test
    Test2.test
  rescue => exception
    p exception
  end
  stale = watcher.watch
  if stale
    Dir.glob("**/*").each { |f| load(f) }
  end
end

So now, if you change the test2 method, you will see the difference. But if you do so for test1, it won't be reloaded. Using require makes it impossible for you to reload this particular file.

This will also work with autoload, but you must remove the constant first.

Conclusion

Now you know how to make your own hot reload. And, of course, you can find the most effective way to do it.

The Spring Gem has 2 strategies if I am not mistaken. First one the simplest is Polling, otherwise, it uses the listen gem developed by Thibaut Guillaume Gentil.